Análise microestrutural de revestimentos aplicados para o reparo de turbinas hidráulicas erodidas por cavitação
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Data
2018-07-27
Autores
Gomide, Danilo Antônio
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Universidade Estadual Paulista (Unesp)
Resumo
A cavitação ocorre inerente ao processo de produção de energia elétrica nas Usinas Hidrelétricas, ocasionando perda progressiva de massa, afetando o desempenho e potência obtidos das turbinas hidráulicas, sendo necessário o seu reparo. Visando minimizar desgaste utiliza-se uma técnica de engenharia de superfície que é aplicar uma camada de material resistente a cavitação. Dentre esses materiais destacam-se os aços inoxidáveis com cobalto, por causa de sua dureza, baixa energia de falha de empilhamento e capacidade de endurecimento por deformação. As deposições desses materiais são realizadas por processos de soldagem, como a soldagem a arco elétrico com atmosfera gasosa ou por soldagem com arame tubular, devido ao baixo custo, elevada taxa de deposição, qualidade de acabamento superficial e baixa taxa de diluição. Diante deste contexto, esse trabalho foi realizado para analisar a camada de aço inoxidável austenítico com cobalto e verificar a influência da camada de amanteigamento. Sendo assim, foram produzidas duas séries de amostra, cada uma composta por três corpos de provas, sendo diferenciadas pela energia de soldagem utilizadas no processo, obtidas pela variação da corrente de soldagem, 98 A, 143 A e 198 A. Na primeira série foi simulado uma condição de reparo das turbinas hidráulicas, utilizando como metal base o aço ASTM A36, sendo depositado sobre ele o arame AWS E70-S6, como camada de preenchimento, posteriormente foi aplicado o arame AWS E309-T1, camada de amanteigamento, e por último foi depositado o aço inoxidável austenítico com cobalto, como revestimento. Na segunda série as amostras foram produzidas sem a camada de amanteigamento. A microestrutura foi analisada por macrografia e microscopia eletrônica de varredura. Os resultados mostraram boa interação metalúrgica entre as diferentes camadas e microconstituintes correspondentes aos investigados na literatura. A microanálise por EDS demonstra a baixa diluição do processo de soldagem utilizado. Os ensaios de difração de raio-X foram capazes de identificar as estruturas de cada camada, como a estrutura CFC no material resistente a cavitação. O ensaio de microdureza instrumentado demostra a capacidade de o revestimento com cobalto resistir à cavitação e que é dispensável a camada de amanteigamento.
Cavitation occurs inherently to the process of production of electric power in the Hydroelectric Power Plants, causing progressive loss of mass, affecting the performance and power obtained from the hydraulic turbines, being necessary its repair. In order to minimize wear, a surface engineering technique is used to apply a layer of cavitation resistant material. Among these materials stand out the cobalt stainless steels, because of their hardness, low stacking failure energy and strain hardenability. The depositions of these materials are carried out by welding processes, such as Gas Metal Arc Welding or by Flux Cored Arc Welding, due to the low cost, high deposition rate, surface finish quality and low dilution rate. Considering this context, this work was carried out to analyze the austenitic stainless steel layer with cobalt and to verify the influence of the buttery layer. Therefore, two series of samples, each composed of three test bodies, were differentiated by the welding energy used in the process, obtained by the variation of the welding current, 98 A, 143 A and 198 A. In the first group, a hydraulic turbine repair condition was simulated using ASTM A36 steel as the base metal. The AWS E70-S6 wire was deposited on it as a fill layer, after which the AWS E309-T1 wire was applied. and finally the austenitic stainless steel with cobalt was deposited as a coating. In the second group of samples were produced without the buttery layer. The microstructure was analyzed by macrography and scanning electron microscopy. The results showed good metallurgical interaction between the different layers and microconstituents corresponding to those investigated in the literature. Microanalysis by EDS demonstrates the low dilution of the welding process used. The X-ray diffraction tests were able to identify the structures of each layer, such as the CFC structure in the cavitation resistant material. The instrumented microhardness test demonstrates the ability of the cobalt coating to withstand cavitation and that the buttery layer is dispensable.
Cavitation occurs inherently to the process of production of electric power in the Hydroelectric Power Plants, causing progressive loss of mass, affecting the performance and power obtained from the hydraulic turbines, being necessary its repair. In order to minimize wear, a surface engineering technique is used to apply a layer of cavitation resistant material. Among these materials stand out the cobalt stainless steels, because of their hardness, low stacking failure energy and strain hardenability. The depositions of these materials are carried out by welding processes, such as Gas Metal Arc Welding or by Flux Cored Arc Welding, due to the low cost, high deposition rate, surface finish quality and low dilution rate. Considering this context, this work was carried out to analyze the austenitic stainless steel layer with cobalt and to verify the influence of the buttery layer. Therefore, two series of samples, each composed of three test bodies, were differentiated by the welding energy used in the process, obtained by the variation of the welding current, 98 A, 143 A and 198 A. In the first group, a hydraulic turbine repair condition was simulated using ASTM A36 steel as the base metal. The AWS E70-S6 wire was deposited on it as a fill layer, after which the AWS E309-T1 wire was applied. and finally the austenitic stainless steel with cobalt was deposited as a coating. In the second group of samples were produced without the buttery layer. The microstructure was analyzed by macrography and scanning electron microscopy. The results showed good metallurgical interaction between the different layers and microconstituents corresponding to those investigated in the literature. Microanalysis by EDS demonstrates the low dilution of the welding process used. The X-ray diffraction tests were able to identify the structures of each layer, such as the CFC structure in the cavitation resistant material. The instrumented microhardness test demonstrates the ability of the cobalt coating to withstand cavitation and that the buttery layer is dispensable.
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Palavras-chave
Aço inoxidável com cobalto, Revestimento, Processos de soldagem, Caracterização microestrutural, Reparo de turbinas hidráulicas, Stainless steel with cobalto, Coating, Welding processes, Microstructural characterization, Repair of hydraulic turbines